Conventional detergent powders contain relatively large quantities of sodium tripolyphosphate (STP). This material is an excellent detergency builder because of its calcium binding power, and in spray-dried powders it also plays another important role: the intermeshing small needle-like crystals of STP hexahydrate provide an excellent matrix for the powder, capable of holding labile ingredients and forming the basis of a powder having excellent flow properties, low compressibility and low tendency to cake. In recent years environmental objections to inorganic phosphates in waste waters have prompted detergent manufacturers to replace STP wholly or partially by non-phosphate builders such as sodium carbonate, sodium nitrilotriacetate or sodium aluminosilicate, but these materials do not, in general, possess an ability comparable to that of STP to contribute to the structure of a spray-dried powder.
EP No. 221 776A (Unilever), published on May 13, 1987, describes and claims novel porous materials consisting of small crystals, comparable to those of STP, interspersed with small pores. One such material, crystal-growth-modified Burkeite, is prepared by drying (preferably spray-drying) a slurry containing sodium carbonate and sodium sulphate in an appropriate ratio and a crystal growth modifier, added to the slurry not later than the sodium carbonate so as to influence the growth of crystals of the double salt Burkeite. The use of crystal-growth-modified Burkeite as the base for a spray-dried detergent powder is described, for example, in Examples 16-23 of the aforementioned European specification. Example 23 describes a powder containing STP as the principal builder and structurant. The powder of Example 23 was prepared by slurrying together all ingredients and spray-drying.
It has now been discovered that spray-dried detergent powders containing crystal-growth-modified Burkeite or similar materials display better particle structure if produced by a method involving the preparation of two separate slurries. Powders containing STP show an additional benefit in that breakdown of STP during spray-drying is also reduced when the method of the present invention is used.